The cells are thin, flattened, contain little cytoplasm, the discoid nucleus is located in the center (Fig. 8.13). The edges of the cells are uneven, so that the surface as a whole resembles a mosaic. There are often protoplasmic connections between adjacent cells, due to which these cells are tightly connected to each other. Squamous epithelium is found in the Bowman capsules of the kidneys, in the lining of the alveoli of the lungs, and in the walls of the capillaries, where, due to its thinness, it allows the diffusion of various substances. It also forms a smooth lining of hollow structures such as blood vessels and heart chambers, where it reduces friction from flowing fluids.
cuboidal epithelium
It is the least specialized of all epithelia; as its name indicates, its cells are cuboidal and contain a centrally located spherical nucleus (Fig. 8.14). If you look at these cells from above, you can see that they have five- or hexagonal outlines. Cube epithelium lines the ducts of many glands, such as salivary glands and pancreas, as well as the collecting ducts of the kidney in areas that are not secretory. The cubic epithelium is also found in many glands (salivary, mucous, sweat, thyroid), where it performs secretory functions.
Columnar epithelium
These are tall and rather narrow cells; due to this shape, there is more cytoplasm per unit area of the epithelium (Fig. 8.15). Each cell has a nucleus located at its base. Secretory goblet cells are often scattered among the epithelial cells; according to its functions, the epithelium can be secretory and (or) suction. Often on the free surface of each cell there is a well-defined brush border formed by microvilli which increase the absorptive and secreting surfaces of the cell. Columnar epithelium lines the stomach; mucus secreted by goblet cells protects the gastric mucosa from the effects of its acidic contents and from digestion by enzymes. It also lines the intestines, where again mucus protects it from self-digestion and at the same time creates a lubricant that facilitates the passage of food. In the small intestine, digested food is absorbed through the epithelium into the bloodstream. Columnar epithelium lines and protects many of the renal tubules; it is also part of the thyroid gland and gallbladder.
Ciliated epithelium
The cells of this tissue are usually cylindrical in shape, but carry numerous cilia on their free surfaces (Fig. 8.16). They are always associated with goblet cells that secrete mucus, which is propelled by the beating of cilia. The ciliated epithelium lines the oviducts, the ventricles of the brain, the spinal canal, and the respiratory tract, where it facilitates the movement of various materials.
Pseudo-stratified (multi-row) epithelium
When considering histological sections of this type of epithelium, it seems that the cell nuclei lie at several different levels, because not all cells reach the free surface (Fig. 8.17). However, this epithelium consists of only one layer of cells, each of which is attached to the basement membrane. Pseudostratified epithelium lines the urinary tract, trachea (pseudostratified cylindrical), other respiratory tracts (pseudostratified cylindrical ciliated) and is part of the mucosa of the olfactory cavities.
characteristic morphological features epithelial tissues
Epithelial tissues are a set of differons of polarly differentiated cells that are tightly adjacent to each other, located in the form of a layer on the basement membrane; they lack blood vessels and very little or no intercellular substance.
Functions. Epithelium covers the surface of the body, secondary cavities of the body, the inner and outer surfaces of the hollow internal organs, form the secretory sections and excretory ducts of the exocrine glands. Their main functions are: delimiting, protective, suction, secretory, excretory.
Histogenesis. Epithelial tissues develop from all three germ layers. Epithelia of ectodermal origin are predominantly multilayered, while those developing from the endoderm are always single-layered. From the mesoderm, both single-layer and stratified epithelium develop.
Classification of epithelial tissues
1. Morphofunctional classification takes into account the structural features and functions performed by one or another type of epithelium.
According to the structure of the epithelium are divided into single-layer and multilayer. Main principle this classification is the ratio of cells to the basement membrane (Table 1). The functional specificity of single-layer epithelium is usually determined by the presence of specialized organelles. So, for example, in the stomach, the epithelium is single-layer, prismatic, single-row glandular. The first three definitions characterize the structural features, and the last one indicates that the epithelial cells of the stomach perform a secretory function. In the intestine, the epithelium is single-layered, prismatic, single-row bordered. The presence of a brush border in epitheliocytes suggests a suction function. In the airways, in particular in the trachea, the epithelium is single-layer, prismatic, multi-row ciliated (or ciliated). It is known that cilia this case play a protective role. Stratified epithelium perform protective and glandular functions.
Table 1. Comparative characteristics of single-layer and stratified epithelium.
|
SINGLE-LAYER EPITHELIUM |
MULTILAYER EPITHELIUM |
|
All epithelial cells are in contact with the basement membrane: |
Not all epithelial cells are in contact with the basement membrane: |
|
1) single layer flat; 2) single-layer cubic (low prismatic); 3) single-layer prismatic (cylindrical, columnar) It happens: |
1) multilayer flat non-keratinizing contains three layers of different cells: basal, intermediate (spiky) and superficial; 5 layers: basal, spiny, granular, shiny and horny; the basal and spiny layers make up the growth layer of the epithelium, since the cells of these layers are capable of dividing. |
Ontophylogenetic classification (according to N. G. Khlopin). This classification takes into account from which embryonic primordium this or that epithelium developed. According to this classification, epidermal (skin), enterodermal (intestinal), colognephrodermal, ependymoglial and angiodermal types of epithelium are distinguished.
So, for example, the epithelium of the skin type covers the skin, lines oral cavity, esophagus, non-glandular chambers of a multi-chamber stomach, vagina, urethra, border section of the anal canal; epithelium of the intestinal type lines the single-chamber stomach, abomasum, intestines; the epithelium of the whole nephrodermal type lines the body cavities (mesothelium of the serous membranes), forms the tubules of the kidneys; ependymoglial type of epithelium lines the ventricles of the brain and the central canal of the spinal cord; angiodermal epithelium lines the cavities of the heart and blood vessels.
For single-layer and multilayer epithelium, the presence of special organelles - desmosomes, semi-desmosomes, tonofilaments and tonofibrils is characteristic. In addition, single-layer epithelium can have cilia and microvilli on the free surface of cells (see the Cytology section).
All types of epithelium are located on the basement membrane (Fig. 7). The basement membrane consists of fibrillar structures and an amorphous matrix containing complex proteins - glycoproteins, proteoglycans and polysaccharides (glycosaminoglycans).
Rice. 7. Scheme of the structure of the basement membrane (according to Yu. K. Kotovsky).
BM, basement membrane; WITH - Light plate; T - dark plate. 1 - cytoplasm of epitheliocytes; 2 - core; 3 - hemidesmosomes; 4 - keratin tonofilaments; 5 - anchor filaments; 6 - plasmolemma of epitheliocytes; 7 - anchoring filaments; 8 - loose connective tissue; nine - Hemocapillary.
The basement membrane regulates the permeability of substances (barrier and trophic function), prevents the invasion of the epithelium into the connective tissue. The glycoproteins contained in it (fibronectin and laminin) promote the adhesion of epithelial cells to the membrane and induce their proliferation and differentiation in the process of regeneration.
By location and function of the epithelium are divided into: superficial (cover the organs from the outside and from the inside) and glandular (form the secretory sections and excretory ducts of the exocrine glands).
Surface epithelium are border tissues that separate the body from external environment and participate in the exchange of substances and energy between the body and external environment. They are located on the surface of the body (integumentary), mucous membranes of internal organs (stomach, intestines, lungs, heart, etc.) and secondary cavities (lining).
glandular epithelium have a pronounced secretory activity. Glandular cells - glandulocytes are characterized by a polar arrangement of organelles general meaning, well-developed EPS and the Golgi complex, the presence of secretory granules in the cytoplasm.
Process functional activities glandular cell, associated with the formation, accumulation and secretion of a secret outside of it, as well as the restoration of the cell after secretion is called secretory cycle.
In the process of the secretory cycle, the initial products (water, various inorganic substances and low molecular weight) enter the glandulocytes from the blood. organic compounds: amino acids, monosaccharides, fatty acids, etc.), from which, with the participation of organelles of general importance, a secret is synthesized and accumulated in cells, and then, by exocytosis, is released into the external ( exocrine glands ) or internal ( Endocrine glands ) environment.
The release of secretion (extrusion) is carried out by diffusion or in the form of granules, but can also be done by converting the entire cell into a common secretory mass.
Regulation of the secretory cycle is carried out with the participation of humoral and nervous mechanisms.
Epithelial regeneration
Different types of epithelium are characterized by high regenerative activity. It is carried out at the expense of cambial elements, which divide by mitosis, constantly replenishing the loss of wearing out cells. Glandular cells that secrete according to the merocrine and apocrine type, in addition, are able to maintain their vital activity not only through reproduction, but also through intracellular regeneration. In the holocrine glands, constantly dying glandulocytes are replaced during the secretory cycle by dividing stem cells located on the basement membrane (cellular regeneration).
Animal epithelial tissue forms single-layer or multi-layer layers covering the internal and external surfaces of any organism.
epithelial cells are connected with each other a small amount of cementing substance, consisting mainly of carbohydrates, and special ligaments - intercellular contacts. The epithelium is underlain by a basement membrane consisting of intertwining collagen fibers enclosed in a matrix. The term membrane should not be confused with cell membranes, which we discussed in Chap. 5; here it simply means a thin layer. The matrix does not interfere with diffusion. Since epithelial cells are not supplied with blood vessels, oxygen and nutrients reach them by diffusion from the lymphatic vessels located in the intercellular spaces. Nerve endings can penetrate the epithelium.
Function of epithelial tissue is to protect the underlying structures from mechanical damage and from infection. With constant mechanical stress, this tissue thickens and keratin is irradiated, and in those areas where the cells are desquamated due to constant pressure or friction, cell division occurs at a very high rate, so that lost cells are quickly replaced. The free surface of the epithelium is often highly differentiated and performs absorptive, secretory or excretory functions, or contains sensory cells and nerve endings specialized to perceive stimuli.
The epithelial tissue is divided into several types depending on the number of cell layers and on the shape of individual cells. In many parts of the body, cells of different types are mixed with each other, and then the epithelial tissue can be difficult to attribute to any particular type.
Simple epithelium
squamous epithelium
squamous epithelial cells thin and flattened. They are flattened so that the core forms a bulge. The edges of the cells are uneven.
As it is clearly seen in the picture cell surface. Neighboring cells are tightly connected to each other by special contacts. Squamous epithelium is found in the Bowman capsules of the kidneys, in the lining of the alveoli of the lungs, and in the walls of the capillaries, where, due to its thinness, it allows the diffusion of various substances. It also forms the lining of hollow structures such as blood vessels and heart chambers, where it reduces friction when fluids flow.
cuboidal epithelium
It is the least specialized of all epithelia. His cells have a cubic shape and contain a spherical core located in the center. If you look at these cells from above, you can see that they have a five- or hexagonal outline. Cubic epithelium lines the ducts of many glands, such as the salivary glands and pancreas, as well as the proximal and distal renal tubules and the collecting ducts of the kidney in areas where they are not secretory.
cuboidal epithelium also found in many glands - salivary, mucous, sweat, thyroid - where it performs secretory functions.
Columnar epithelium
The cells of this epithelium tall and rather narrow; due to this shape, there is more cytoplasm per unit area of the epithelium. Each cell has a nucleus located at its basal end. Among the epithelial cells, goblet cells are often scattered; according to their functions, the cylindrical epithelium can be secretory and (or) suction. Often on the free surface of each cell there is a well-defined brush border formed by microvilli, which increase the suction and secretory surface of the cell. Columnar epithelium lines the stomach; the mucus secreted by the goblet cells protects the gastric mucosa from the effects of acidic contents and from digestion by enzymes. It also lines the intestines, where again mucus protects the intestinal walls from self-digestion and at the same time creates a lubricant that facilitates the passage of food. In the small intestine, digested food is absorbed through this epithelium into the bloodstream. Columnar epithelium lines and protects many of the renal tubules; it is also found in the thyroid gland and gallbladder.
Ciliated epithelium
The cells of this epithelium usually have a cylindrical shape, but carry numerous cilia on their free surfaces. They are always associated with goblet cells that secrete mucus flowing through the beating of cilia. The ciliated epithelium lines the inside of the oviducts, the ventricles of the brain, the spinal canal and the respiratory tract (trachea, bronchi and bronchioles), ensuring the movement of various substances through them. For example, in the airways, cilia move mucus up into the throat, which makes it easier to swallow solid food. Mucus traps bacteria, dust, and other small particles, preventing them from entering the lungs.
Pseudo-stratified (multi-row) epithelium
When considering histological sections of this epithelium it seems that the cell nuclei lie at different levels, because not all cells reach the free surface of the tissue. However, this epithelium consists of only one layer of cells, each of which is attached to the basement membrane. Pseudostratified epithelium lines the urinary tract and respiratory tract (trachea, bronchi, bronchioles, where it is covered with cilia and consists of cylindrical cells).
First of all, epithelial tissues are divided into single-layer and stratified epithelium. A unilayer epithelium is an epithelium in which all cells lie on a basement membrane. In the stratified epithelium, the cells lie in several layers, but only the bottom row of cells touches the basement membrane.
Single layer epithelium.
Single layer epithelium, consisting of cells of the same shape and size, is called single-row. However, in cases where a single-layer epithelium consists of cells of unequal shape and size, such an epithelium is called multi-row. Single-row epithelium may consist of prismatic, cuboidal, or squamous cells. In this regard, there are single-layer squamous epithelium, single-layer cubic epithelium, single-layer columnar epithelium.
Single layered squamous epithelium- mesothelium, lines all the serous membranes (pleura, peritoneum, cardiac membrane), develops from the mesoderm. Cells have a polygonal or somewhat irregular shape. The boundary between the cells is uneven, due to which the protrusions of the cell membrane of one cell protrude into the recesses of another cell. Cell borders are detected only when treated with silver. Each cell contains one, less often several, flattened nuclei. The cytoplasm is granular and contains vacuoles. Electron microscopy reveals small microvilli on the surface of mesothelial cells. The cytoplasm contains all common organelles: mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.
Mesothelium, covering the serous membranes, prevents the formation of connective tissue adhesions that occur during inflammatory diseases. In addition, through the mesothelium, the process of absorption of substances from the serous cavities is carried out. These processes of absorption occur most intensively along the periphery of the cell. During regeneration, mesothelial cells increase their planar dimensions and move to the wound surface. Cell reproduction occurs by mitosis.
Single layered cuboidal epithelium lines the tubules of the kidneys, small bronchi, ducts of the glands, etc. In various organs, this epithelium performs various functions: in the kidneys - absorption, in the glands - secretory, etc. In embryogenesis, this epithelium develops from the mesoderm and endoderm. Each cell of this epithelium has approximately the same height and width. Sometimes there are microvilli on the apical surface of cuboidal epithelial cells.
Single layered columnar epithelium- is located in the middle section of the digestive tract, in the uterus and oviducts, excretory ducts of the glands (liver and pancreas). This epithelium develops from different germ layers: from the endoderm (intestinal epithelium), from the mesoderm (epithelium of the tubules of the kidneys, vas deferens). The functional significance of this epithelium varies in different organs. So, the epithelium of the stomach secretes mucus, which promotes the digestion of food and protects the mucous membrane from chemicals. The intestinal epithelium is involved in absorption processes. In all cells of the prismatic epithelium, polar differentiation is pronounced. Cell nuclei are elliptical in shape and lie in the basal part of the cell. Organelles are located above the nucleus. Special structures can form on the apical surface: microvilli in the intestinal epithelium, cilia in the uterine epithelium.
Single layered epithelium lines the mucous membrane of the airways. This epithelium develops from the endoderm and mesoderm.
In a single layered epithelium, all cells lie on the basement membrane. In this case, the shape and size of the cells are not the same. There are several types of cells in this epithelium. Prismatic cells (ciliated)- the tops of these cells make up the surface of the epithelial layer and often have ciliated cilia. The basal part of the cells is narrowed, and the apical part is expanded. Insertion cells cubic and spindle-shaped, located between the prismatic ones. goblet cells- these are cells that secrete mucus (mucin) on the surface of the epithelium, which protects it from mechanical, chemical and infectious influences. Basal cells- These are low cells, lie on the basement membrane and belong to the cambial cells, which divide and differentiate into ciliated and goblet cells. In addition, this epithelium contains endocrine cells, which carry out local regulation of the muscle tissue of the bronchi. Due to the fact that these cells have a different shape, their nuclei lie at different levels and form several rows, therefore such an epithelium is called multi-row. The single-layer multi-row ciliated epithelium of the airways, due to the oscillation of the cilia, contributes to the removal of dust particles.
Stratified epithelium- This is an epithelium, which consists of several layers of cells. In this case, only the lower layer of cells lies on the basement membrane. There are stratified squamous keratinized epithelium, stratified squamous non-keratinized epithelium and stratified stratified transitional epithelium.
Stratified squamous nonkeratinized epithelium covers the cornea of the eye, the mucous membrane of the oral cavity, esophagus, etc. The cells of this epithelium are located in several layers. The cells of the lower layer, lying directly on the basement membrane, have a cylindrical shape. These cells are poorly differentiated and divide by mitosis. Due to these cells, all other layers are replenished. Therefore, this layer (basal) is called the germ layer. In the next layers, the cells flatten and acquire processes that wedged between the underlying cells. These cells are called prickly. The closer to the surface, the more flattened the cells become. The surface cells are flat, these cells also contain tonofibrils.
Stratified squamous keratinized epithelium- is surface layer skin (epidermis). In contrast to the non-keratinizing epithelium, in this epithelium, cells are transformed into horny scales, which lie on the surface in the form of a layer. The transition to horny scales is carried out gradually, therefore, many layers are found in the composition of the keratinizing epithelium. The cells of this epithelium are called keratinocytes.
The deepest layer is a layer of high prismatic cells lying on the basement membrane - this basal layer. cell membrane in the basal part of the cells gives deep finger-like protrusions penetrating into the dermis. Due to this layer, the strength of the bond with the underlying tissues is ensured. Here are the stem cells of differon keratinocytes. In addition, melanocytes are located in this layer, the cytoplasm of which contains a large number of melanin pigment granules concentrated around the nucleus. There is also a small number of intraepidermal macrophages (Langerhans cells. A layer of spiny cells is located above the basal cells. These cells are characterized by the presence of a large number of processes (spines). Keratinosomes appear in the cytoplasm of these cells, which are granules containing lipids. These granules are secreted into macrophages and melanocytes also lie here.Melanocytes, with the help of pigment, create a barrier that prevents the penetration of ultraviolet rays into the body.Langerhans cells (macrophages) are involved in immune reactions and regulate the reproduction of keratinocytes, forming together with them "proliferative units" Then there are 2-3 layers of flat cells (keratinocytes), in the cytoplasm of which keratohyalin protein granules appear, which indicates the beginning of the keratinization process.In addition to keratohyalin, the cells of the granular layer contain filaggrin proteins (rich in histidine), involucrin, keratol inin, loricrin. These proteins are involved in the processes of keratinization. This layer is called the granular layer. Then comes a shiny layer, represented by flat cells impregnated with the protein ellaidin. The surface layer consists of horny scales, which are air bubbles surrounded by keratin protein. Between the scales there is a cementing substance, a product of keratinosomes, rich in lipids, which gives the layer a waterproofing property. The outermost horny scales lose contact with each other and constantly fall off the surface of the epithelium. They are replaced by new ones - due to reproduction, differentiation and movement of cells from the underlying layers. Due to this, the epidermis is completely renewed every 3-4 weeks. The significance of the keratinization process lies in the fact that the stratum corneum formed in this process is resistant to mechanical and chemical influences, poor thermal conductivity and impermeability to water and many water-soluble toxic substances.
Stratified transitional epithelium. This epithelium got its name due to the fact that it can change its structure. Transitional epithelium lines the renal pelvis, mucosa of the ureters, bladder, and other organs of the urinary tract. If you take the wall of the bladder filled with urine (stretched), and consider the structure of its epithelium, you can see a two-layer epithelium. At the same time, the basal layer of cells is represented by cells of a cubic shape. The superficial cells are also cuboidal, but much larger. The epithelium of the bladder, which is in a collapsed state, has a different structure. Due to the fact that the surface of the basement membranes, as it were, decreases, some of the cells of the basal layer do not fit on it and are displaced into an additional layer, but retain their connection with the basement membrane with a narrow stalk.
Thus, the transitional epithelium changes its structure depending on the functional state of the organ, i.e. change in its volume.
According to the ability to secrete, epithelial tissues are divided into 2 main types: integumentary (non-glandular) and glandular (secretory).
Glandular or secretory epithelium. This is the epithelium that secretes a secret onto its free surface. For example, the mucous membrane of the stomach, intestines, bronchi, urinary organs is always moistened with a secret produced by epithelial cells. Secretory epitheliocytes are characterized by high degree development of the endoplasmic reticulum, mitochondria and the Golgi apparatus i.e. organelles directly involved in the secretion process. Secretory granules are present at the apical pole of these cells. In addition, glandular cells are characterized by the presence of intracellular capillaries, which are folds of the plasmalemma.
In some cases, glandular cells are concentrated in organs that specialize in secretion - glands. Glands are formed during embryogenesis from epithelial cells growing into the underlying connective tissue. All glands in our body are divided into endocrine and exocrine. Endocrine glands are glands that secrete their secret directly into the blood or lymph (pituitary gland, pineal gland, thyroid gland, etc.). Exocrine glands are glands that secrete their secret into the cavity or on the surface of the skin (salivary, sweat, sebaceous, prostate, etc.).
exocrine glands. Exocrine glands are unicellular and multicellular. The only example of single-celled glands in the human body are goblet cells. Multicellular glands consist of two main parts: specialized cells that synthesize the secret (secretory or terminal) and a system of tubes (tubules) through which the secret moves (excretory ducts).
Thus, the exocrine glands consist of end sections and excretory ducts. The shape of the end sections distinguish: alveolar, tubular and alveolar-tubular glands. According to the structure of the excretory duct exocrine glands are divided into simple and complex. Simple glands are glands in which the excretory duct does not branch (sweat glands). Complex glands are characterized by the presence of a branching excretory duct (liver, pancreas, salivary glands). According to the structure of the end section distinguish between branched and unbranched glands.
Exocrine glands are different the nature of the secret. In this regard, there are protein (serous) glands (parotid, pancreas), mucous (goblet cells), protein-mucous (submandibular, sublingual) and sebaceous (sebaceous glands of the skin), saline (lacrimal, sweat).
The protein terminal sections consist of prismatic secretory cells, the cytoplasm of which stains basophilically, which is due to the content of free ribosomes and those associated with the endoplasmic reticulum. The rounded nucleus lies on the basal pole. At the apical pole there are numerous granules of an immature secret - a zymogen, which is a vesicle surrounded by a membrane containing a secret intended for excretion.
The mucous terminal sections consist of large irregularly shaped cells, the nuclei of which are flattened and located at the basal pole closer to the basement membrane. The cytoplasm is light and filled with vesicles containing mucus.
Protein-mucous (mixed) end sections consist of mucous cells, on top of which there is an accumulation of a group of protein cells, resembling a crescent moon in shape and called the protein crescent.
Exocrine glands differ from each other not only in the nature of the secretion secreted, but also according to the method (mechanism) of secretion of this secret. Merocrine glands (salivary) secrete their secret through the plasmalemma in the form of bubbles surrounded by a membrane, while the integrity of the plasma membrane is not violated. With the apocrine type of secretion, partial destruction of the apical part of the secretory cells (sweat glands of the axillary region, mammary glands) is possible. However, a number of researchers do not recognize this type of secretion. In the holocrine glands, in the process of secretion, the destruction and death of the entire cell occurs, i.e., the cells die and are destroyed, thus forming a secret that is pushed out through the hair follicles and lubricates the hair. The only example of this type of secretion is the sebaceous glands of the skin. At the same time, the restoration of dead cells is carried out due to poorly differentiated cells located on the basement membrane.